I just killed a keg of lager that I brewed out of a yeast starter, and I decided to replace it with the lager which I brewed off that starter and which I had been lagering for 2+ months.

That original batch had a defect. It tasted sort of rubbery or chemical-y. I chalked it up to under-pitched lager yeast.

With the new batch, before racking to the keg, I sampled about half a cup. It was delicious, clean, and had no detectable off flavors at all. During racking, I heard a dripping sound in the keg. I realized I had not purged the keg before racking so I got concerned about oxidation. Unfortunately I couldn't let go of the autosiphon and let it drop into the yeast cake in order to go over to the keg and manually place the end of the siphon hose on the bottom of the keg. I called my wife over to help, but by the time she got there, the dripping had stopped.

2 days later, I pulled a sample off the keg and it has the same damn defect!!! I wouldn't really call it wet cardboard but I'm almost positive the issue is oxidation. I've had it in 3 of my kegged beers now (they've all been lightish beers) and I've never had this defect when bottling. It's really starting to cheez me off.

So how do I prevent this in the future? Just purge the keg before racking and ensure the siphon hose touches the bottom of the keg before I start?

Don't know much about kegging, but that flavor you speak of definately sounds like oxidation. I hate that for you. I'd imagine purging will help, but just keeping the siphon hose head under the siphoning beer will help quite a bit as well I think.